a. Field of Invention
This invention relates to cardiac pacemakers and defibrillators. More particularly, it relates to an apparatus and method for classifying cardiac rhythms and providing appropriate electrical stimulation to the heart.
b. Description of the Prior Art
If an implantable device which is to provide electrical stimulation to the heart is to be truly useful, then it must be able to sense the electrical activity of the heart, classify cardiac rhythms, and determine what electrical therapy would be most appropriate.
Various methods and apparatus have been proposed for sensing and classifying cardiac rhythms. The cardiac waveform may be monitored for determining AA, AV, VA, and VV intervals. It is also possible to monitor the morphology of the waveform. In addition, hemodynamic sensors have been used. Generally, morphology and hemodynamic sensors are used as gating functions.
The difficulty with most such classification apparatus and methods is that they are not accurate. Further, the use of morphology or hemodynamic condition as a sensing or gating function does not provide for a fine enough classification of cardiac rhythm.
U.S. Pat. No. 5,086,772 entitled Arrhythmia Control System Employing Arrhythmia Recognition Algorithm, assigned to the assignee of the present invention, utilizes an arrhythmia recognition algorithm to detect and distinguish among bradycardia, sinus rhythm, superventricular tachycardia and ventricular fibrillation, and deliver therapy in the form of electrical energy to cardiac tissue to revert selected arrhythmias and restore normal sinus rhythm. The techniques described therein are generally more accurate than most because the rhythm classification part of the method disclosed therein provides numerical outputs and rhythm categories which are not overlapping and are mutually exclusive from one another, so that there is no doubt of the rhythm category being outputted at any given time.
The difficulty with all the above techniques of heart rhythm classification is that they assume a good quality input signal that enable a reliable and consistent classification to be made. It is common for the input signal (whether it is electrical, haemodynamic or other measure of the heart's function) to be of insufficient quality for consistent and accurate classification, and thus cause these systems to deliver incorrect therapy to the patient. An example of this is with some schemes of antitachycardia pacing which require a very accurate measurement of the heart beat interval. In the situation where the heart's electrogram is such that a sufficiently accurate measurement cannot be made, it is safer to use some other form of therapy that does not require this accuracy.